A new recombinant form of the plasmid-encoded protein pgp3 from C. trachomatis, serotype D, was purified by ion exchange column chromatography and shown to be suitable for quantitative immunoassay on clinical samples in an ELISA format. Since initial attempts of developing a similar assay with a SDS-denatured pgp3-fusion protein failed, the results suggest that for anti-pgp3 antibody detection, antigen conformation is important.
The function of the 7.5 kb plasmid, pCT, of Chlamydia tyachomatis is still unknown. However in fact that this DNA element appears to be strongly conserved in C. trachomatis (both in terms of its presence in essentially all isolates and in terms of is genetic structure) suggests that pCT may provide some, important, and perhaps advantageous, function to the chlamydial cell during its natural host infection. Recently, an open reading frame of pCT (ORF3; Comanducci et al., Plasmid, 23:149-154, 1990) was expressed in E. coli as a recombinant fusion protein (Comanducci et al., J. Gen. Microbiology, 134:1083-1092, 1993). The expression system used added a 11-kDa N-terminal polypeptide of the MS2-bacteriophage polymerase to the 28-kDa polypeptide (pgp3) encoded by ORF3. The resulting 39-kDa product was used to show that pgp3 epitopes can be recognized on Western blots by antibodies present in sera from patients with chlamydial infections, but not in control sera from healthy donors. Following this observation, we subsequently tried to develop a serological test more suitable than the immunoblotting technique for obtaining reproducible and quantitarive data from large numbers of clinical samples. We now report that an enzyme-linked immunoassay based on a new recombinant form of the pgp3 protein, can be used for assessing the prevalence of pgp3 antibodies in people with C. trachomatis infections.